Chain Transfer - Polymer Engineering Questions and Answers

This set of Polymer Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Chain Transfer”.

1. What does the effect of chain transfer on R_p depend on?
a) re-initiation rate constant, k_i^’
b) monomer concentration
c) k_i^’/k_p
d) none of the mentioned
View Answer

Answer: c
Explanation: The effect of chain transfer on R_p depends upon the value of re-initiation constant relative to the value of propagation constant. If k_i^’<< k_p, then chain transfer agent would play the role of an inhibitor or retarder and R_p would be adversely affected.

2. How is the value of average degree of polymerization defined, considering the chain transfer mechanism?
a) ratio of overall rate of polymerization to total rate of production of pairs of chain ends
b) ratio of total rate of production of chain ends to overall rate of polymerization
c) ratio of overall rate of polymerization to rate of formation of pairs of chain ends by chain transfer
d) ratio of rate of formation of pairs of chain ends by chain transfer to overall rate of polymerization
View Answer

Answer: a
Explanation: The average degree of polymerization is redefined from the kinetic view point as the ratio of overall rate of polymerization to total rate of production of pairs of chain ends.

3. What is the value of chain transfer constant for a transfer agent?
a) rate constant for chain transfer reaction, k_tr
b) product of k_tr and k_p
c) ratio of k_tr to k_p
d) none of the mentioned
View Answer

Answer: c
Explanation: The chain transfer constant C is defined as the ratio of rate constant, k_tr for chain transfer reaction to the propagation rate constant.

4. What is the value of intercept, when (1/X_n) is plotted against R_p, if only monomer transfer has a significant occurence?
a) 0
b) C_M
c) C_I
d) [I]
View Answer

Answer: b
Explanation: In the absence of any solvent in polymerization system, the expression for (1/X_n) is arranged as-
1/X_n = [(y+2)/(y+1)](k_t/k_p²)(R_p/[M]²) + C_M + C_I [I]/[M] When only monomer transfer takes place, the intercept on 1/X_n axis gives a measure of monomer transfer constant, C_M.

5. What is the value of intercept and slope of the straight line, when the term [1/X_n– [(y+2)/(y+1)](k_t/k_p²)(R_p/[M]²)] is plotted against the ratio [I]/[M], when both monomer and initiator transfer are considered?
a) C_M, C_I
b) C_I, C_M
c) 0, C_I
d) none of the mentioned
View Answer

Answer: a
Explanation: The plot of [1/X_n – [(y+2)/(y+1)](k_t/k_p²)(R_p/[M]²)] vs [I]/[M] gives a straight line, whose intercept gives a direct measure of CM and slope gives the value of initiator transfer constant, CI.

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6. How does the value of solvent transfer constant, C_S vary in the solvents with increasing substitution at the α-carbon?
a) increases
b) decreases
c) constant
d) none of the mentioned
View Answer

Answer: a
Explanation: The reactivity increases with the increasing substitution at the α-carbon, which leads to increasing values of C_S in the order benzene < toluene < ethyl benzene < isopropyl benzene.

7. Which of the following solvents is not useful for controlling the degree of polymerization?
a) carbon tetra-bromide
b) pentaphenyl ethane
c) t-butyl mercaptan
d) toluene
View Answer

Answer: d
Explanation: The other three solvents exhibit high chain transfer activity; they surpass styrene in reactivity towards chain radical and have C_S values greater than unity. Toluene, on the other hand, has a very low C_S value to control the degree of polymerization.

8. What is the slope of the linear plot of log [S] v/s log [M] in significant solvent transfer mechanism?
a) C_M
b) C_S
c) k_tr,S
d) C_I
View Answer

Answer: b
Explanation: The ratio of rate of consumption of the transfer agent and the rate of consumption of monomer can be written as, (d[S]/dt)/(d[M]/dt) = k_tr,S[M]/k_p[S] = C_S [M]/[S] d log[M]/d log [S] = C_S
Hence for this system, the linear plot of log [M] against log [S] renders the slope of solvent transfer constant, C_S.

9. What is the effect of polymer transfer on the polymer product formed by radical polymerization?
a) becomes less crystallizable
b) becomes mechanically weak
c) becomes less resistant to foreign agents
d) all of the mentioned
View Answer

Answer:d
Explanation: The polymer transfer leads to the chain branching which makes it less crystalizable, weak in mechanical properties and less resistant to heat, solvents and chemicals.

10. For polymerization of vinyl acetate with hydrogen peroxide at 60ᵒC, the reciprocal of number average degree of polymerization is given by,
1/X_n = 1.75*10^-4 + 14.0R_p + 5.4*10⁴R_p²
What is the value of monomer transfer constant?
a) 1.75*10^-4
b) 0.875*10^-4
c) 5.4*10⁴
d) 14
View Answer

Answer: a
Explanation: According to the expression for reciprocal of average degree of polymerization, we have
1/X_n = C_M + (k_t/k_p²)(R_p/[M]²) + C_I(k_t/k_p² fk_d)(R_p²/[M]³)
Comparing the given equation with this one, we get CM as 1.75* 10^-4.

11. Consider the expression for reciprocal of average degree of polymerization, when styrene is polymerized with t-butyl hydroperoxide at 60ᵒC-
1/X_n = 0.6*10^-4+10.0R_p+1.73*10⁴ R_p²
Calculate the value of initiator transfer constant, if the monomer concentration is 8.35 mol/l and the value of the term k_p² fk_d/k_t= 3.29*10^-9.
a) 0.035
b) 0.07
c) 0.063
d) 0.0175
View Answer

12. The transfer constant to the solvent for polymerization of of styrene in toluene at 60ᵒC is 0.125*10^-5. How much dilution is required to decrease the molecular weight by 4 times when 1/X_n0 is 3.5*10^-4?
a) 75
b) 42
c) 84
d) 7
View Answer

Answer: c
Explanation: To decrease molecular weight by 4 times, 1/X_n becomes 4/X_n0,
4/X_n0 = 1/X_n0 + C_I [S]/[M] Substituting the values, [S]/[M] = 84. Thus, dilution factor is 84.

13. The molecular weight of the polymer, when styrene is polymerized at 60ᵒC, is halved by diluting it with ethyl benzene. What is the value of transfer constant to the solvent, if the dilution factor is 7 and the term 1/X_n0 is 4.8*10^-4?
a) 0.335
b) 0.68
c) 1.5
d) 0.82
View Answer

Answer: b
Explanation: We have the expression for solvent transfer,
1/X_n = 1/X_n0 + C_I [S]/[M] When molecular weight is halved, 1/X_n becomes 2/X_n0, then substituting the values and [S]/[M]= 7, we get C_I as 0.68.

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